Relocation of inactive X chromatin during S phase: the role of CIZ1

X chromosome inactivation (XCI) allows the establishment of gene dosage compensation between male and female mammals. The long non-coding RNA Xist recruits polycomb repressive complexes (PRC), forming transcriptionally silent heterochromatin, involving accumulation of H2AK119Ub1 and H3K27me3. The nuclear matrix protein CIZ1 is required for retention of Xist and co-localizes with enriched H3K27me3 marks at the inactive X chromosome (Xi), allowing Xi to be used as a model to study CIZ1. EdU incorporation and thymidine arrest, the new alternative assay developed here, have allowed for the first time the requirements of Xi relocation to be explored.